1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device having a wide viewing angle characteristic.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 7-120728 discloses a display mode called "ASM mode" (Axially Symmetric Aligned Microcell Mode), where liquid crystal molecules are oriented in axial symmetry in each pixel, in order to improve the viewing angle characteristic of the display. In a display device of this mode, a plurality of liquid crystal regions are formed by phase separation from a mixture of a liquid crystal material with a positive dielectric anisotropy and a photocurable resin, so that the liquid crystal molecules in each liquid crystal region are oriented in axial symmetry.
Japanese Laid-open Patent Publication No. 8-341590 discloses a liquid crystal display device including: a pair of substrates; and a liquid crystal layer interposed between the pair of substrates, wherein: the liquid crystal molecules in the liquid crystal layer have a negative dielectric anisotropy; the liquid crystal layer includes a plurality of liquid crystal regions; the liquid crystal molecules are oriented in a direction substantially perpendicular to the pair of substrates in the absence of an applied voltage; and the liquid crystal molecules in each liquid crystal region are oriented in axial symmetry in the presence of an applied voltage. This liquid crystal display device operates in a normally black mode, and provides a higher contrast ratio than that of an ASM mode liquid crystal display device which operates in a conventional normally white mode. Moreover, the liquid crystal display device can be produced relatively easily.
FIGS. 1A and 1B illustrate a liquid crystal display device of Japanese Laid-Open Patent Publication No. 8-341590, wherein FIG. 1A is a cross-sectional view and FIG. 1B is a plan view thereof. In the liquid crystal display device, a pair of substrates (e.g., a glass substrates 4 and 8) are provided to oppose each other with a predetermined gap therebetween. A liquid crystal layer 6 of liquid crystal molecules with a negative dielectric anisotropy is interposed between the glass substrates 4 and 8. A signal electrode 9 of a transparent conductive film (e.g., ITO) is formed in a stripe pattern on the inner surface (closer to the liquid crystal layer 6) of the glass substrate 4. A vertical alignment layer 22 of polyimide, or the like, is provided over the signal electrode 9 so as to cover substantially the entire surface of the glass substrate 4. A color filter (not shown) and a black matrix (not shown) are provided on the inner surface (closer to the liquid crystal layer 6) of the glass substrate 8. A signal electrode 10 of a transparent conductive film (e.g., ITO) is formed in a stripe pattern over the color filter and the black matrix. The striped signal electrode 10 is arranged to cross the striped signal electrode 9, thereby forming a pixel at each intersection therebetween. The color filter (not shown) includes RGB color layers for each pixel. The black matrix (not shown) has a pattern to cover the gap between adjacent color layers of the color filter (not shown). A plurality of partition walls 17 are provided on the glass substrate 8, with pillar-like spacers 20 being provided selectively and regularly on some of the partition walls 17, thereby defining a plurality of liquid crystal regions 15. A vertical alignment layer 21 of polyimide, or the like, is provided on the side surfaces of the pillar-like spacers 20 and on a portion of the glass substrate 8 on which the pillar-like spacer 20 is not provided. Thereafter, the pair of substrates are attached together with the predetermined gap therebetween into which a liquid crystal material is injected, thereby producing a display cell.
FIG. 2 is a schematic cross-sectional view showing a part of the liquid crystal display device of Japanese Laid-Open Patent Publication No. 8-341590. FIG. 2 illustrates the orientation of liquid crystal molecules 11 (11a, 11b and 11c) in the liquid crystal layer 6 in the vicinity of the partition wall 17 along the periphery of the liquid crystal region 15 in the absence of an applied voltage. The partition wall 17 is provided on the signal electrode 10 which is provided on the glass substrate 8. The vertical alignment layer 21 covers the top and side surfaces of the partition wall 17 and the surface of the signal electrode 10.
In the absence of an applied voltage, the liquid crystal molecules 11a along the side surface of the partition wall 17 are subject primarily to the anchoring force of the part of the vertical alignment layer 21 along the side surface of the partition wall 17, and thereby oriented in a direction substantially perpendicular to the side surface of the partition wall 17. The liquid crystal molecules 11b along the signal electrode 10 are subject primarily to the anchoring force of the part of the vertical alignment layer 21 along the signal electrode 10, and thereby oriented in a direction substantially perpendicular to the substrate 8. The liquid crystal molecules 11a at the corner of the side surface of the partition wall 17 and the substrate 8 are subject to both the anchoring force from the part of the vertical alignment layer 21 along the side surface of the partition wall 17 and the anchoring force from the part of the vertical alignment layer 21 along the signal electrode 10. Consequently, the liquid crystal molecules 11c may suffer from disturbance in their orientation, which results in the liquid crystal molecules not being uniformly oriented in a single direction. Due to the disturbance in the orientation of the liquid crystal molecules, the liquid crystal molecules 11c may produce birefringence, leading to light leakage. The light leakage may cause the contrast ratio of the display device to decrease in a black display, thereby deteriorating the display quality.
As will be discussed later in greater detail, the partition walls 17 define the position and the size of the respective liquid crystal regions 15 of the liquid crystal layer. Based on the anchoring force from the side surface of the partition wall 17, the liquid crystal molecules 11 in each liquid crystal region 15 are controlled to exhibit axially symmetric orientation in the presence of an applied voltage (white display). Conventionally, the partition wall 17 requires a sufficient height with respect to the thickness of the liquid crystal layer (or the cell gap) in order for the side surface of the partition wall 17 to provide the anchoring force and thereby to maintain a stable axially symmetric orientation of the liquid crystal molecules 11 in the liquid crystal region 15. With an insufficient height of the partition wall 17, the axially symmetric orientation of the liquid crystal molecules 11 may not be controlled sufficiently, thus failing to obtain the stable axially symmetric orientation. In such a case, the axially symmetric orientation may be destroyed, resulting in non-uniformity in a produced display.
The presence of the partition wall 17 in the liquid crystal layer 6 has presented the following problems. First, the partition wall 17 may present an obstruction to a liquid crystal injection process, thereby increasing the injection time and thus the production cost of the display device. Particularly, in a large-screen liquid crystal display device, there may occur a distribution in the composition ratio of the liquid crystal material across the liquid crystal panel, due to a phenomenon called "chromatographic phenomenon", thereby resulting in non-uniformity in a produced display.
When the display is viewed from an angle inclined from the direction normal to the display panel, some or all of incident light having passed through the liquid crystal region 15 may be blocked by the partition wall 17, thereby reducing the optical transmission and thus the brightness of the display device. This phenomenon becomes more pronounced as the partition wall 17 is taller and/or the view angle is inclined more from the direction normal to the display panel. When the display is viewed from an inclined angle, the incident light having passed through the liquid crystal region 15 may pass through the liquid crystal layer 6 just above the partition wall 17. In such a case, the optical transmission may be improved by effectively utilizing those liquid crystal molecules 11 which exist just above the partition wall 17 so that they contribute to a produced display. However, only a voltage effectively lower than a liquid crystal driving voltage is applied through those liquid crystal molecules 11 just above the partition wall 17, and thus such liquid crystal molecules 11 provide little or no contribution to a produced display in the presence of an applied voltage.